The invention relates to an exposure device with a lamp, a condenser arrangement, a light modulator, a field lens and a projection objective.
Such an exposure device which is used for exposing printing plates with ultra-violet light is for example known from DE 195 45 821 A1. With this the sample to be imaged is broken down into part pictures by way of a computer and the part pictures are brought after one another onto an electronically controllable light modulator, for example a through-beamed LCD screen or a micro-mirror arrangement. The exposure device is then bit by bit moved over the printing plate to be exposed, wherein the light modulator in each case is controlled with the associated part picture.
With the use of micro-mirror arrangements (DMD, Digital Mirror Device) with which minute mirrors with edge lengths of a few micrometers are arranged on the surface of an electronic chip and can be individually tilted by electronic control, the tilting angle of the micro-mirror is limited. In order, according to the control of the micro-mirrors, to guide the incident beam into the projection objective or next to this, between the incident and reflected beams an acute angle must be maintained. On account of these circumstances the incident and emergent beam bundles compete for the same spacial angle region. Thus the cross sections of the optical elements arranged in this region and thus also the cross sections of the beam bundle are limited.
It is the object of the invention to specify an exposure device with as high an exposure intensity as possible with a uniform illumination of the object to be exposed.
The solution of this object lies in the fact that the light modulator consists of a reflecting micro-mirror arrangement in front of which the field lens is arranged in a manner such that the beam path runs through the field lens onto the micro-mirror arrangement and after modulation and reflection at acute angles once again runs through the field lens.
By way of the combination of the micro-mirror arrangement with the field lens one may optimally exploit the advantages of the micro-mirror arrangement, specifically a high throughput of light, since the doubly acting field lens reduces the cross sections of the incident and of the emergent beam bundle and thus in spite of the low spacial angle region ensures a high light throughput without shadings.
The light throughput may be still improved in that the beam cross sections of the beam bundle incident and reflected on the micro-mirror arrangement are formed oval and with their longer transverse extensions are arranged essentially perpendicular tot he plane formed by the incident and emergent direction. Thus the beam bundles in the direction where they compete for the same spacial angle have a smaller extension, and in the direction running perpendicularly to this, in which there prevails no competition, a larger extension, which has the result of a larger throughput of light.
The invention may even be further improved in that in the beam path of the incident beam bundle, after the condenser, a convergent lens with a large diameter and subsequently a convergent lens with a small diameter are arranged such that the beam bundle converges from the large convergent lens towards the small convergent lens and again diverges, in the further course progresses up to the field lens and after the reflection at the micro-mirror arrangement and renewed passage through the field lens is corrected such that the whole bundle is transported from the projection objective. At the same time preferably the optical path between the smaller convergent lens and the field lens is approximately equal to the optical path between the field lens and the projection objective. With this arrangement the small convergent lens is located in the vicinity of the projection objective, where the incident beam competes with the reflected beam for the same spacial angle. Exactly in this region however the cross section of the incident beam bundle as a result of the combination of the larger and smaller convergent lens is advantageously at the lowest.
The part pictures to be exposed have in a simple embodiment variation the shape of a rectangle. With this shape however there is the danger that the corners of the rectangle obtain a lower illumination intensity than its centre. An non-uniform exposure may however not be accepted. Although the lateral lengths of the rectangle may be reduced, however this would lead to an increased total exposure time, since for the exposure of the large surfaced object many more individual exposures must be carried out.
In order to exploit the region of the optimal exposure intensity as much as possible, therefore in another preferred embodiment variation it is suggested that the surface pieces are formed as hexagons. Such hexagons may be put together into a surface free of gaps just as rectangles. With this the corners of the hexagon have a smaller distance to the surface centre than the corners of the equal surfaced rectangle. Thus with the same number of individual exposures a more uniform illumination of the object to be exposed can be achieved.
In order to form the device more compact as a whole with known CtP-UV exposure devices for printing plates the optical axis from the lamp up to the convergent lens behind the collector is aligned essentially at right angles to the optical axis of the projection objective, wherein the latter is preferably arranged vertically in order to expose the usually horizontally arranged printing plate. In order with such an exposure device to maintain the acute angle between the beam bundle incident on the micro-mirror arrangement and the reflected beam bundle, for guiding the beam two plane mirrors are required, wherein in the beam path after the large convergent lens there is arranged a first plane mirror from which the beam path reaches through the small convergent lens and where appropriate through further plane mirrors onto a last plane mirror which is arranged directly next to the projection objective and guides the light at an acute angle to the objective axis through the field lens onto the micro-mirror arrangement from where it falls into the projection objective.
With UV exposure devices for printing plates gas condenser discharge lamps are used, which for technical reasons only function when their longitudinal axis is aligned vertically. These lamps produce an oval light spot whose longitudinal axis is likewise aligned vertically. Such a vertical light spot is however reflected in such a manner by the previously described mirror arrangement that its longitudinal axis, with respect to the competition situation, is unfavourably directed in the spacial angle region of the projection objective. For avoiding this disadvantage it is suggested that in the beam path between the first plane mirror and the last plane mirror further mirrors are arranged in a manner such that the longitudinal axis of the light spot picture in the projection objective is directed perpendicularly the plane tentered from the direction of incidence and emergence of the reflection at the micro-mirror arrangement. In this manner with the acute-angled reflection caused by the micro-mirror arrangement one obtains a higher light throughput whilst avoiding shadings by the optical elements taking part.
Since the printing plates to be exposed are often not completely alinged planely, one usually tracks the exposure device accommodated in an exposure head, wherein the exposure head as a whole must be displaced in the vertical direction. In order to reduce the mass to be moved on displacing and thus to permit a quick displacement, it is suggested that a first componentry consisting of the lamp and the condenser is mechanically separated from a second componentry consisting of the subsequent optical elements up to the projection objective and that the second componentry for compensating distance variation of the projection objective to the object to be exposed is displaceably arranged perpendicular to the surface of the object. By way of this measure the particularly large mass of the lamp and of the condenser are grouped together in a first unit and are separated from the second unit to be moved.
The uniform exposure of the individual surface pieces may be still further improved in that in the beam path between the condenser and the micro-mirror arrangement there is arranged a prism for compensating the non-uniform illumination intensity as a result of the oblique incidence of light on the micro-mirror arrangement. On account of the oblique incidence of the beam bundle there arises in the plane of the micro-mirror arrangement a distortion of the exposing beam bundle. For example an originally square surface element due to the distortion obtains the shape of trapezoid in the illumination beam path. The prism provided for compensation produces then an opposing distortion which compensates the effect as a result of the oblique incidence of light.
In another embodiment form the non-uniform illumination intensity on the micro-mirror arrangement is compensated by an electronically produced masking. At the same time the micro-mirrors are controlled such that the temporal average reflectivity in the surface regions with a high illumination intensity is smaller than in surface regions with a lower illumination intensity.